Electrodeposition of lead and lead alloys



nited 2,751,341 Patented June 19, 19.56

2,751,341 ELECTRODEPOSITION F LEAD AND LEAD ALLOYS Clarence F. Smart, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application September 22, 1952, Serial No. 310,927

Claims. (Cl. 204-43) This is a continuation-in-part of my co-pending application S. N. 291,563 filed June 3, 1952.

This invention relates to improvements in the electrodeposition of lead and alloys of lead.

it has heretofore been very ditficult to obtain a smooth, lustrous, adherent electrodeposited coating of lead or of a lead alloy. Electrodeposited lead or lead alloy coatings have generally been dull and unattractive. In prior lead plating baths the dissolved lead salts have been substantially completelyionized. As a result, with the large number of available lead ions, lead plated out of the bath too rapidly and unevenly producing a rough and uneven deposit of lead having poor surface quality.

. l have now discovered that a smooth, adherent coating of lead or of a lead alloy may be electrodeposited from an aqueous bath containing certain addition agents. In the plating of lead or alloys of lead according to my invention, the preferred addition agent is the tetra sodium salt of ethylene diamine acetic acid. Under some conditions, the mono, di and hi sodium salt of ethylene diamine tetra acetic acid may also be used. If desired, the potassium salt may be used instead of the sodium salt.

Generally, an aqueous bath having a high throwing power, suitable for the electrodeposition of lead or lead alloy coatings according to my invention comprises an aqueous solution of lead and an alkali metal salt of ethylene diamine tetra acetic acid. The addition agent of my invention permits the electrodeposition of an improved lead coating by controlling the ionization of the dissolved lead in the plating bath. As a result, only a relatively small concentration of lead ions exists in the bath at a given time. Lead may be electrodeposited from such a bath evenly and smoothly to form an adherent coating on the cathode. As the lead ions are depleted, the addition agent permits the ionization of more lead to provide 'a steady supply of lead ions. Hence, it will be seen that in a bath employing my addition agent it is possible to incorporate an amount of lead greatly in excess of the amount conventionally used in a lead plating bath without affecting the high quality of the electrodeposited coating since the addition agent of my invention will not permit the excess lead to ionize, but maintains it in reserve in the solution until needed. The addition agent does not 'pla'te out of solution at the cathode. It is depleted only by mechanical dragout and may be replenished as needed.

The addition agent of my invention generally may be incorporated in an acid or alkaline plating bath in any amount. However, most satisfactory results have been obtained when the addition agent is present in an amount at least equal to about two and one half times the amount of dissolved lead in the bath. While any greater amount of the addition agent may be used, up to the limit of its solubility in the bath if desired, when the ratio of addition agent to dissolved lead in the bath falls much below 2.5, the surface quality of the lead electrodeposit Wlll tend to depreciate.

Lead or lead alloys may be plated m any deslred thickness from baths embodying my invention. When the lead coating is intended as an outer protective coating, thicknesses up to 0.0005 inch may be plated although a coating of about 0.0002 inch in such an application is preferred. A coating of lead or of a lead alloy may have a layer of antimony or antimony alloy electrodeposited on its surface if desired. As an undercoat for further electrodeposited coatings it is preferred to use a bright lead coating having a thickness not in excess of 0.0001 inch. Using a bath containing my addition agent, lead or lead alloys may be electrodeposited on any conventional cathode material such as iron, steel, copper, copper plated iron or steel, nickel, cadmium, zinc, etc.

Any soluble lead salt or combination of soluble lead salts may be used as a source of lead in a bath embodying my invention. For example, lead acetate, lead sulfate, lead chloride, lead fluoborate, or lead oxide (PbO) may be used. It will be understood, of course, that these are illustrative only of some of the suitable lead salts and that other soluble lead compounds may be used as well. Some lead salts which are relatively insoluble in water, for example, as is lead sulfate, may be used in a bath embodying my addition agent because of an increased solubility in the solution. The use of lead oxide (PbO) gives very beneficial results because when dissolved in the bath it supplies lead ions without adding any foreign anions to the solution. Hence, if desired, an excess of lead oxide may beadded to the bath. The addition agent will control the ionization of the lead oxide as it dissolves. In this manner, the ionization of the lead oxide is controlled and the excess of lead oxide present, either dissolved or undissolved in the bath, provides a ready reserve of lead as it is needed Without adversely affecting the bath composition.

A specific and illustrative example of a suitable alkaline plating bath for the electrodeposition of lead coatings is the following where the quantities expressed are per liter of water:

50 to grams of sodium potassium tartrate (Rochelle salt),

20 to 40 grams of tetra sodium salt of ethylene diamine tetraacetic acid,

5 to 15 grams of lead acetate.

It will be understood, of course, that other ingredients may be substituted for those mentioned above; for example, the lead acetate may be replaced by lead oxide (PbO) or any other suitable lead salt. The sodium potassium tartrate (Rochelle salt) may be replaced by another tartrate, citrate, acetate or other anode corrosion promoters if desired. Generally a tartrate would be preferred in an alkaline bath because it is more soluble.

A bath having a composition within the above ranges provides excellent anode corrosion when anodes of lead are used. If it is desiredto electrodeposit lead using inert anodes, maintaining the supply of lead ions by separate additions of a lead salt, the tartrate, citrate, acetate or other anode corrosion promoter may be omitted entirely Without affecting the high quality electrodeposit obtained.

An alkaline lead bath is particularly advantageous when it is desired to provide a protective coating on metals which are readily attacked by acid plating baths,

Baths having compositions within the above mentioned ranges operate most satisfactorily at a pH of from 9 to ll. While the addition agent of my invention, when dissolved in water, provides an alkaline solution, if desired, sodium hydroxide or other basic material may be added as needed to maintain the desired pH.

Alkaline baths formed according to my invention generally may be operated at current densities of from 10 to 20 amperes per square foot using 1 to 3 volts. It is preferred to operate the baths at room temperature; i. e. 70-75 'F. However, the baths may be operated at any desired temperature, limited generally only by the practical considerations of minimizing heating costs and evaporation losses.

, A specific and illustrative example of an acid plating bath embodying my invention is the following where the quantities expressed are per liter of water: dibasic ammonium citrate 40-120 grams, tetra sodium salt of ethylene diamine tetra acetic acid 20-40 grams, lead oxide -15 grams.

Dissolving the dibasic ammonium citrate in water provides a solution having a pH of about 4-5. However, in order to overcome the alkalinity of my addition agent when dissolved in water, sulfuric, acetic or hydrochloric acid may be added to the bath as needed to control the pH which, for most satisfactory results, should be maintained between 4 and 5.

It will be understood, of course, that instead of the dibasic ammonium citrate, other anode corrosion promoters; i. e. citrates, tartrates or acetates may be used as well. Generally the citrates or acid tartrates are preferred as anode corrosion promoters in an acid bath, however, because of their greater solubility. While baths of the above composition provide excellent anode corrosion using lead anodes, if it is desired to operate the bath using an inert anode, maintaining the lead concentration by additions of lead compounds, the citrate, tartrate or other anode corrosion promoter may be omitted entirely without affecting the high quality of the electro-deposit obtained.

A plating bath employing the addition agent of my invention may also be used in the electrodeposition of smooth coatings of lead alloys, for example, as in the plating of an alloy of lead and indium, lead and antimony, etc. If it is desired to plate an alloy of lead,

generally it is only necessary to add to the bath a suitable salt of the metal it is desired to plate with lead and to maintain an adequate supply of my addition agent in the bath so that the ionization will be properly controlled. A specific example, illustrative of a suitable bath for the electrodeposition of a lead-indium alloy, is the following in which the quantities are per liter of water: lead acetate 5-15 grams, indium chloride 5-15 grams, tetra sodium salt of ethylene diamine tetra acetic acid 20-40 grams, sodium potassium tartrate (Rochelle salt) -l20 grams. If desired, indium hydroxide or other suitable indium salt may replace the indium chloride. Indium hydroxide has the advantage that when dissolved in the bath it does not add any foreign anion to the solution. It

will, of course, be understood that the lead acetate may 'be replaced by any other suitable lead salt such as lead oxide (PbO), lead fluoborate or lead chloride. A bath having the above composition provides excellent anode corrosion using anodes of lead and indium or an anode of a lead-indium alloy such as an anode formed of an alloy composed of 4 to 10% indium and 90 to 96% lead. If it is desired to operate the bath using inert anodes, the

tartrate or other anode corrosion promoter, of course,

may be omitted without affecting the quality of the electrodeposit.

A smooth coating of a lead-indium alloy may be electrodeposited from a bath having the above composition either as an acid or an alkaline bath. Most satisfactory results, however, are obtained when the bath is either quite acid; i. e. having a pH of about 3 or is quite alkaline; i. e. having a pH of about 10. While the above bath may be operated over a wide range of temperatures it generally is preferred to operate the bath within a range from F. to about 125 F. Most satisfactory results are obtained when the lead-indium bath is operated using about three volts with a current density of from 10 to 20 amperes per square foot.

In preparing a bath employing the addition agent of my invention, the various ingredients may be dissolved in 'water in any desired sequence. However, in some .4 cases where some of the ingredients are not too soluble in water it may be advantageous to first mix the addition agent with the water to provide a solution in which the other ingredients are more easily dissolved.

If desired, conventional wetting agents, buffers, etc. may be added to baths embodying the addition agent of my invention. However, very satisfactory results are obtained without any added materials since my addition agent not only is a very effective wetting agent but also acts as a buffer in stabilizing the bath composition.

Various changes and modifications of the embodiments of the invention described herein may be made by those skilled in the art without departing from the spirit and principles of the invention.

I claim:

1. A lead plating bath consisting essentially of an aqueous solution of sodium potassium tartrate, lead acetate, and tetra sodium salt of ethylene diamine tetra acetic acid in an amount sufficient to complex lead in solution.

2. A plating bath consisting essentially of an aqueous solution of the following ingredients in which the concentrations are per liter of water: 50 to 120 grams of sodium potassium tartrate, S to 15 grams of lead acetate, 20 to 40 grams of tetra sodium salt of ethylene diamine tetra acetic acid.

3. An aqueous electroplating bath consisting essentially of an aqueous solution of lead oxide (PbO), indium hydroxide and an alkali metal salt of ethylene diamine tetra acetic acid in an amount suflicient to complex the lead in solution.

4. An aqueous electroplating bath suitable for the deposition of lead-indium alloy coatings which consists essentially of an aqueous solution of lead oxide (PbO), indium hydroxide, tetra sodium salt of ethylene diamine tetra acetic acid and sodium potassium tartrate, said tetra sodium salt being present in an amount sufiicient to complex lead in solution.

5. An electroplating bath consisting essentially of an aqueous solution of the following ingredients in which the concentrations are per liter of water: 5 to 15 grams of lead oxide, 5 to 15 grams of indium hydroxide, 20 to 40 grams of tetra sodium salt of ethylene diamine tetra acetic .acid, 40 to 120 grams of sodium potassium tartrate.

6. An electroplating bath for the electrodeposition of lead coatings which consists essentially of an aqueous solution having dissolved therein dibasic ammonium citrate, lead acetate and tetra sodium salt of ethylene diamine tetra acetic acid in an amount suflicient to complex lead in solution.

7. An electroplating bath consisting essentially of an .aqueous solution of the following ingredients in which the concentrations are per liter of water: 40 to 120 grams of ammonium citrate (dibasic), 20 to 40 grams of alkali metal salt of ethylene diamine tetra acetic acid, 5 to 15 grams of lead acetate.

8. An aqueous electroplating bath suitable for the deposition of bright lead coatings containing 10 grams per liter of lead oxide (PbO), grams per liter of sodium potassium tartrate and 30 grams per liter of tetra sodium salt of ethylene diamine tetra acetic acid.

9. An electroplating bath for electrodeposition of lead which consists essentially of an aqueous solution containclaim 9 to which has been added a soluble compound of indium.

(References on following page) References Cited in the file of this patent FOREIGN PATENTS UNITED STATES PATENTS 731,102 Germany Feb. 3, 1943 1,837,835 Pinner et a1 Dec. 22, 1931 130,302 Great Bmam July 1919 2,458,839 Dyer et a1. Jan. 11, 1949 5 OTHER REFERENCES 2,474,092 Liger June 21, 1949 Senderofi: Metal Finishing, vol. 48 (September 1950), pages 71-78. 

9. AN ELECTROPLATING BATH FOR ELECTRODEPOSITION OF LEAD WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING AT LEAST ONE ANODE CORROSION PROMOTER OF THE GROUP CONSISTING OF CITRATES, TARIRATES AND ACETATES, AT LEAST ONE MEMBER OF THE GROUP OF LEAD COMPOUNDS CONSISTING OF LEAD OXIDE, LEAD ACETATE, LEAD CHLORIDE, LEAD FLUOBORATE, AND LEAD SULFATE AND AN ALKALI METAL SALT OF ETHYLENE DIAMINE TETRA ACETIC ACID IN AN AMOUNT SUFFICIENT TO COMPLEX LEAD IN SOLUTION. 